A light emitting diode (LED) is a semiconductor device constituted mainly by group III-V compound semiconductor materials, for instance. Such semiconductor materials have a characteristic of converting electricity into light. Hence, when a current is applied to the semiconductor materials, electrons therein would be combined with holes and release excessive energy in a form of light, thereby achieving an effect of luminosity.
Generally speaking, since the lattice mismatch between gallium nitride (GaN) and sapphire substrate is approximately 16%, a large quantity of defects are generated at the lattice interface, and thus causing a drastic decay in the light emitting intensity. The amount of defects is unavoidable during the growth process of LED. However, when the emitted wavelength of light from the LED is 450 nm, it is known that lattice stress is released around the defects and forms self-assembled indium-riched regions. Therefore, when carriers move to the defects, the carriers are likely to capture by the self-assembled indium-riched regions, thus forming the so-called localized effect. Since the quantum confinement effect of the self-assembled indium-riched regions is capable of increasing the carrier recombination rate, therefore, even though the GaN LED is limited by the high defect density, a certain degree of luminous intensity is still maintained at the 450 nm wavelength of light.
When the luminous wavelength of the LED gradually shifts from blue to the ultraviolet wavelengths of light, due to the concentration of indium decreasing gradually in the active layer, the self-assembled indium-riched regions are also correspondingly lessened. Consequently, the carriers in the LED are likely to move to the defect areas and generate non-radiative recombination, thereby drastically decreasing the luminous intensity of the LED at the ultraviolet wavelengths. As a result, manufacturers in the pertinent art endeavour to develop LED with satisfactory luminous efficiency.